A gas ratio control device of this type has become known from DE 41 11 139 A1 (corresponding to U.S. Pat. No. 5,335,652; U.S. Pat. No. 5,335,652 is hereby incorporated by reference). Anesthetic gas, laughing gas in this case, is sent in the prior-art gas ratio control device into an inlet chamber of a control valve via an anesthetic gas feed line, and then to a fresh gas outlet from an outlet chamber of the control vane via an anesthetic gas-adjusting valve and a first measuring resistance. Oxygen is also sent as an additional gas via an oxygen feed line, an oxygen-adjusting valve and a second measuring resistance to the fresh gas outlet, where the two gas flows merge.
Dynamic pressures, which are sent as a differential pressure to a proportional element, are generated by the gas flows at the measuring resistances, and the proportional element controls the control valve in the anesthetic gas feed line according to the value of the difference between the two dynamic pressures so that the anesthetic gas flow, i.e., the flow of laughing gas, does not exceed a certain value in relation to the flow of oxygen.
The proportional element has a first pressure chamber to receive the dynamic pressure generated at the first measuring resistance, and a second pressure chamber to receive the dynamic pressure generated at the second measuring resistance. A diaphragm, which is deflected in proportion to the pressure prevailing in the pressure chambers, is located in each pressure chamber. A ball seat vane in the control valve is actuated via a rod connecting the diaphragms, as a result of which the flow of anesthetic gas from the inlet chamber into the outlet chamber changes.
Difficulties in adjustment arise in the case of forms of anesthesia involving very low flow rates of fresh gas, e.g., an oxygen flow rate of less than 1 L/minute; these difficulties may cause the flow of anesthetic gas through the control vane to be shut off completely or partially, as a result of which the oxygen concentration in the fresh gas may increase up to 100 vol.%. This is due essentially to the fact that the rod leading from the first pressure chamber into the outlet chamber and actuating the ball seat valve must overcome frictional forces at the sealing seat between the first pressure chamber and the outlet chamber. The frictional forces are also increased by the fact that the feed pressure occurring in the anesthetic gas feed line in the worst case may act in the outlet chamber. Since the pressure in the outlet chamber also acts on the sealing seat between the outlet chamber and the first pressure chamber, the frictional force increases as a result. In addition, such diaphragm-controlled valves tend to swing.
A pressure-reducing vane with a closing piston, which is connected to a control piston located in a control cylinder and to a damping piston accommodated in a damping cylinder, has been known from DE 32 19 552 A1. A control pressure actuating the control piston is admitted to the control cylinder. The damping piston divides the damping cylinder into an upper cylinder space and a lower cylinder space, which are connected to one another via a laminar throttle. The sensitivity of the closing piston to suddenly occurring pressure shocks is reduced by the throttled gas exchange between the two cylinder spaces on both sides of the damping cylinder. However, the effectiveness of damping depends on the control pressure actuating the control piston in the prior-an pressure-reducing valve, because the lower cylinder space and the control cylinder are connected to one another via a bypass line. Thus, the pressure equalization via the laminar throttle during the movement of the damping piston is also influenced by the pressure and flow conditions in the bypass line.
To make possible the operation of the prior-an gas ratio control device even at low gas flow rates, attempts were made to bridge over the control valve with a bypass that can be connected (DE-C 38 10 745), but this makes possible only a constant addition of the anesthetic gas to the flow of fresh gas, and the amount added is determined by the throttling point of the bypass line.